Gravel Sound

[markmac]

I've ready numerous posts on the fourms about pumps soundling like they are pumping gravel. The otehr day when the water level was down too much, the float in the skimmer fell enough that it forced all the suction to the main drain. When this happened, the pump becomes very loud and does in fact sound like its pumping gravel. I also notice air bubbles in the pump, that are not there when pumping form the skimmer.

Its a single speed 2HP pump. From what I can tell, the main drain feeds into the skimmer, and then one line from teh skimmer to the pump.

My question is could the bubbles be from a leak, or something else? I notice the pressure at the filter is 25 when running via the skimmer, and then 15 when running the main drain (i.e. the skimmer float locked down).

 

[zea3]

When the water level falls so that the weir does not float freely in the skimmer, the pump will begin sucking air into the system. That is why your pump basket has bubbles. The air can cause pressure to build up in the filter and can cause damage to the grids or filter casing if allowed to continue. You need to bring up the water level in the pool so it is at least 1/2 way up the skimmer opening, and open the air bleed on the filter to let out the excess air.

If you have a valve that cuts off all flow to the skimmer and pulls only from the main drain the bubbles and noise should go away after you bleed the air out of the system. If it is drawing water only from the main drain and you continue to get air bubbles then there may be a suction side air leak.

 

[mas985]

The gravel sound could be cavitation so I would avoid allowing the water to get too low. But the bubbles are either from the weir as zea3 pointed out or it could also be due to a leak in the pump lid. Either way it should go away after the water is returned to normal and the pump draws from both the skimmer and MD.

 

[linen]

I am curious here since I do not think have not heard the "gravel" sound in my brief time spent around pool pumps. My understanding is the gravel sound is typically caused by cavitation that occurs at/on the impeller surface. The water vapor that is created would not show up in the pump basket, but should be downstream of the impeller, so the "air" in the strainer is not due to the potential pump cavitation? Is the air in the basket (however it got there) somehow reducing the net back force on the impeller, which in turn causes the impeller to speed up? Does this in turn cause the water that still makes it to the impeller to then cavitate and cause the "gravel sound"? I thought typical residential motors rotated at 3450 rpm (on high speed) and not faster? Sorry for all of the questions, but I am trying to understand this.

 

[mas985]

Yes that is a lot of questions and I will try to answer all of them.

Cavitation in general is pretty rare in pool pumps but it can happen in larger pumps under high suction conditions. Cavitation cannot be directly observed unless you have a window directly at the location of the cavitation and see the bubbles form and collapse very near the impeller inlet vanes where they occur. The flow rate will tend to move them downstream but they don't make it very far because as water travels through the impeller it gains pressure which immediately collapses the vapor bubbles. It is the collapsing of the vapor bubbles which makes the gravel sound and which also causes damage to the impeller.

The reason cavitation occurs is because the pressure at the impeller is very low and it causes the water to boil. When the pressure gets that low, it is nearly impossible for the pump lid seal to prevent air getting sucked in. So typically, cavitation and air in the pump basket occur side by side which has created the impression that when you see air in the pump basket it means the pump is cavitating when it may not be. However, the opposite is almost always true. When the pump cavitates you will most always see air in the pump basket.

Pumping air (not cavitation bubbles) will actually reduce the load on the impeller which does have a minor effect on the motor slip so a very tiny increase in speed may be noticed. The slip of a typical pool pump motor is around 4% but it does vary slightly with load.

 

[linen]

Mark,

Thanks for the great explanation!

So in the ops case could a restriction say at the floor drain causing increase in suction side vacuum (as possibly suggested by the lower pressure at outlet of the pump...ie at the filter) which would cause the water to be closer to it's vapor pressure when it reaches the impeller, then the impeller causes an additional drop in pressure due to speed of the water over the impeller vanes (Bernoulli's principle) so that voila, we have cavitation?

 

[markmac]

Thanks for all the info. A couple points to clarify.

- from what I can tell, its just the one line from the skimmer to the pump. There is no other line to the pump, so the skimmer and MD have to combine somewhere, and the only thing I see is the two holes in the skimmer.
- from what I can tell, as long as the water level is fine, all the water pulled by the pump is coming from the skimmer, as there is no real water flow that I can feel coming up from the main drain hole at the bottom of the skimmer. I also do not see any leaves,dirt etc pulled toward the MD on the pool floor. Even when forcing only the MD as the source, nothing really pulls to the MD when I sweep it towards it.
- the only way to force the pump to pull from the main drain is to close the skimmer and allow the float to lock, and then the noise starts.
- when the pump is pulling from the skimmer as normal, no bubbles, no leaks at the pump. Everything seems good.
- With the fact the pressure drops, and bubbles appear in the basket when I force all suction to the main drain, does that point to a leak, or a blockage?
- I don't see any real drop in water over time, so would I, if there was a leak in the MD?

One other question is how the system is possibly setup. Could my assumption be right that the main drain goes directly to the skimmer and then to the pump in a combined skimmer/MD single 1.5" line? The pool is probably built in the 80's.

Thanks again for all the info.

 

[mas985]

- from what I can tell, its just the one line from the skimmer to the pump. There is no other line to the pump, so the skimmer and MD have to combine somewhere, and the only thing I see is the two holes in the skimmer.

The main drain is probably plumbed through one of the ports in the skimmer. This is pretty common in a lot of pools although not exactly optimal.

- from what I can tell, as long as the water level is fine, all the water pulled by the pump is coming from the skimmer, as there is no real water flow that I can feel coming up from the main drain hole at the bottom of the skimmer. I also do not see any leaves,dirt etc pulled toward the MD on the pool floor. Even when forcing only the MD as the source, nothing really pulls to the MD when I sweep it towards it.

You won't normally see debris move toward the main drain because the cover distributes the flow rate in such a way as to minimize the velocity and force. If it is an anti-vortex cover, it is even better at distributing the force. So this is not a surprise.

- the only way to force the pump to pull from the main drain is to close the skimmer and allow the float to lock, and then the noise starts.

This is an illusion. The float itself causes the main drain to flow into the skimmer and you won't see or feel this happening without the float installed so there is no way to know if it is flowing. But trust me with the float in and the water at proper level, the main drain does have some flow but it really only needs a little flow anyway. Main drains have only a minor impact on the circulation.

With the fact the pressure drops, and bubbles appear in the basket when I force all suction to the main drain, does that point to a leak, or a blockage?

In a way it is a leakage but it is a leakage that occurs only because of the high suction which is created by the skimmer block. This is not a normal condition and your equipment was not designed to do this so you really shouldn't be forcing the pump to cavitate. Over time it can damage the impeller.

 

[arvil]

Sorta sounds like blockage in the main drain line, that the pump is getting nothing when the skimmer is blocked. Otherwise, the pump should get water from the drain. The pressure at the inlet side of the impeller is the lowest when it can't get water, hence the pressure decreases to the point that cavitation starts. Cavitation is when the water turns to vapor (steam) when the pressure decreases to the point that it can boil at the temperature of the water. In addition, water always has air dissolved in it, the air comes out of solution contributing to the cavitation.
In the case of my AG pool, I can get cavitation from shutting the valve from the skimmer to the pump since that is the only source for the pump. Good luck with the drain.